This invention relates generally to geothermal energy recovery systems and more particularly to a turbine-impeller pump for use in such system.
Except where heated brine has risen of its own accord from the depths to the surface of the earth, the recovery of thermal energy from such heated brine has heretofore been non-competitive with the more conventional sources of energy because of the costs and multiple problems which attach to installations of systems and the related equipment for accomplishing such recovery.
However this source of thermal energy has now become more important because of the increasing costs of conventional forms of energy and therefore the methods of recovering such energy are now being investigated more thoroughly.
In the prior art various types of recovery systems are already known. These systems are generally classified into the following:
1. Air lift systems in which compressed air is injected into the hot brine well which expands and reduces the weight or density of the mixture therein sufficiently to raise the hot brine from the well. These systems are inefficient and require separation of air and brine after the heated brine is raised to the surface.
2. Deep well pumps of the multi-stage bowl type which are directly driven from the surface provide high efficiency in the order of about 80% depending on the size of the unit but there are limitations to such installations because of alignment, expansion, distortion, corrosion and erosion problems.
3. Eductor pumping of hot brine is feasible because there are no rotating elements but these systems are limited as to depth because of the pressures and velocities required. Further the efficiency of these systems is low being in the order of 30%.
4. Submersible electric motor driven pumps have high efficiency in the order of 80% but these systems have sealing problems in respect to the electrical cables due to erosion and temperature damage to the insulation because of the hot brine being pumped.
It is also known that water at the temperature which exist in the acqua perin is capable of dissolving and holding in solution large amounts of solids such as sodium, barium and calcium chloride and sulphates, and minerals such as boron, arsenic, magnesium, etc. The dissolved salts and minerals can reach levels as high as 20 to 30% by weight of these hot brine solutions. Such concentrated hot brine solutions can create major problems to recovery apparatus and systems because they are extremely corrosive and erosive damaging blades, vanes and nozzles of rotating equipment, and further percipitate on the inside of bores, openings, pipes, and collecting chambers of a recovery system and restrict flow of the fluid through such equipment and the system.
Thus, the equipment and systems must not only be capable of moving the hot concentrated brine from whatever depths the same is located but additionally must be capable of handling the fluid withoug damage due to corrosion, erosion, scaling and other problems associated with this type of fluid.
This will require the use of special materials in the equipment or in the systems for such geothermal energy recovery.
In any such equipment and systems, it is preferable to maintain the hot brine in the liquid phase to reduce the precipitation of solids and further to permit return or reinjection of the brine solution; after heat has been recovered therefrom; into the site area adjacent to the area of the supply walls, to eliminate environmental problems which the removal of such hot brine may cause.
The present invention provides an improved geothermal recovery system as it utilizes a turbine-impeller pump which is a shaftless arrangement not subject to thermal distortion. It is hydraulically balanced having a rotor with spaced composite impellers and oppositely disposed driving assemblies so that thrust forces are negligible and can by reason of the composite impellers be driven by the same hot brine fluid medium being pumped which is recirculated from the discharge outlet for the system by a driving pump, at the surface, convenient to the recovery well. The turbine-impeller pump can be placed at any depths the speed is easily regulated as a function of the pressure required for delivering the hot brine from the depth of the earth, speed regulation being a function of the pressure at which the driving medium is supplied to the turbine portion of the turbine-impeller pump.
Further the geothermal energy recovery system utilizing a turbine-impeller pump in accordance with the present invention by reason of its simplicity of structure has relatively low installation cost, ease of regulation, and by reason of its hydraulic balance reduced maintenance.
It is further noted that the geothermal energy recovery system utilizing a turbine-impeller pump in accordance with the present invention will have exceptionally good draw down characteristics as the spaced composite impellers on the rotor of the turbine-impeller pump will permit the hot brine to be drawn to a relatively low level in a supply well during periods of peak load operation and will permit the hot brine to replenish itself from the perculating fluids in the earth during periods when the system is operating at low load requirements.